1. Field of the Invention
This invention relates to the field of infrared imaging devices, and more particularly to a buried channel charge-coupled dual wavelength infrared focal plane imager.
2. Description of Related Art
Infrared detectors are often used in conjunction with missile and night vision systems to sense the presence of electromagnetic radiation having wavelengths of 1-15 .mu.m. Such detectors are often embodied in focal plane imagers in which two-dimensional representations of an image can be obtained by using appropriate scanning techniques. Focal plane imagers may generally be characterized as either being monolithic or hybrid in structure. In monolithic focal plane imagers, the operations of photodetection, charge generation, and charge transfer are performed by a structure designed from a single material. To perform these operations within a single structure, nonolithic focal plane imagers generally comprise a charge-coupled device ("CCD") or an array of MOSFET'S for random access addressing used in conjunction with an infrared sensitive substrate consisting of either a narrow bandgap semiconductor or an extrinsic semiconductor having the appropriate impurity energy level. A narrow bandgap material for infrared detection in the 1-15 .mu.m range is simply a material in which the energy required to excite a valence band electron to the conduction band is relatively low (between nominally 1.0 eV and 0.1 eV for the indicated wavelength ranges).
Charge-coupled devices used with the focal plane imagers described above may generally be characterized as having an elongated MOS tansistor with an extended array of gates between its source and drain. During operation of a p-channel charge-coupled device, a charge packet consisting of a concentration of holes are held in place for a short period of time by application of a negative voltage to one of the gates. If that voltage is then reduced while an adjacent gate is energized, the charge packet migrates to a new position under the adjacent gate. By applying pulses to alternate gates, a sequence of charge packets can be transferred from one or more gates to the drain of the imager. Similar devices can be made from n-channel materials.
One monolithic focal plane imager which incorporates a charge-coupled device is described in U.S. Pat. No. 4,273,596. The imager disclosed in this patent comprises a substrate layer of cadmium telluride upon which four epitaxial layers are grown. The last of the epitaxial layers interfaces with a series of Schotty barrier contact gates which are connected to a charge-coupled device. The fabrication of this type of device is relatively complex, having the inherent drawback of requiring a window layer, a substrate layer, a wide bandgap spillover layer, a narrow bandgap absorber layer, a transfer layer, and a channel layer in addition to the layers which form the gates of the charge-coupled device. Fabrication of the epitaxial layers of this device may require that several of the layers be individually doped and may involve the use of metalorganic chemical vapor deposition procedures.
While the imagers generally described above would be somewhate effective in processing infrared signals, other drawbacks in addition to those already mentioned exist. By not having a buried channel, the constraints placed on the passivant layers of the CCD are generally severe with a simultaneous demand for simple passivation and high quality gate insulator characteristics. Thus, low interface state densities and fixed charge in the insulator must be achievable while also making the detector insensitive to contamination which might otherwise cause premature failure or drift in its electrical characteristics. In addition, the high voltage required to lower the electropotential barrier at the heterojunction between the wide and narrow bandgap materials could produce relatively large tunnel currents. Finally, these devices will not generally detect infrared radiation in more than one specific range in the infrared spectrum.